The invention described herein was made in the course of, or under, a grant from the U.S. Public Health Service.
The present invention relates to apparatus for mechanically inducing inspiration and expiration in a patient. The invention further relates to ventilation/perfusion scans using scintimetric methods for diagnosis of abnormalities in the lungs. More particularly, the present invention relates to a ventilator apparatus which can be instantaneously switched between isolated oxygen/air and oxygen/air/Xenon delivery systems while continuously ventilating the patient and maintaining ventilation parameters constant. As used herein, the term ventilation parameters includes the ventilation rate, inspiratory time, oxygen concentration, end-expiratory pressure, and tidal volume.
Ventilation/perfusion scans utilizing scintimetric methods for diagnosis of abnormalities in the lungs have been available as a diagnostic tool for some time. A ventilation scan is utilized to determine ventilatory patterns in the lungs. A perfusion scan is usually also performed at the same time to determine intrapulmonary shunting.
The combined ventilation/perfusion scan is usually performed in the following manner. During a so-called "wash-in period" the patient breathes a suitable gaseous mixture containing a traceable gas, usually a radioactive isotope such as Xenon.sup.133, until an equilibrium state is reached at which there is a uniform concentration of the traceable gas throughout the lungs. During the wash-in period, a radiation detecting apparatus is utilized to generate a scintrimetric picture of the radioactive isotope as it spreads through the various regions of the lungs. The picture is in the form of an illuminated pattern. During the wash-in period, another radioactive isotope, for example Technetium in the form of microspheres, is injected into the patient's blood stream. The Technetium flows through the intricate network of blood vessels in the lungs. The spread of Technetium throughout the lungs is also shown in the scintimetric picture. By observing the spread of the Xenon gas throughout the lungs, the physician can determine ventilatory patterns. Dead spaces where no ventilation occurs can be located. The pattern formed by the Technetium provides valuable information concerning intrapulmonary shunting.
During a so-called "wash-out period" the patient breathes an oxygen/air mixture until the Xenon gas is substantially eliminated from his or her lungs. The physician gains further valuable information concerning the ventilatory patterns in the lungs by observing the gradual retreat of the radioactive gas. Thus, ventilation/perfusion scanning can provide valuable geographic data on the location of ventilation/perfusion mismatch areas and the response of these areas to treatment. The technique can also be used in clinical research. For example, it may be utilized to demonstrate changes in ventilation/perfusion relationships in human beings and animals in response to various physicological and mechanical conditions.
Heretofore the use of ventilation scans with Xenon gas has been limited to those patients capable of breathing spontaneously and in whom the inspired oxygen concentration has not been critical. This is because known Xenon delivery systems require that the patient be removed from a conventional life-support ventilating apparatus in order to perform a Xenon ventilation scan. For some critically ill patients, even a momentary interruption of mechanical ventilation, or a variation in one or more of the ventilation parameters can result in further health complications and in some instances even death. Therefore, the ventilation/perfusion scanning technique has not been available to patients requiring long term, continuous mechanical breathing assistance.
However, it is well recognized that patients who require mechanical ventilation for life-support have even a greater need for ventilation/perfusion scanning than those who are capable of breathing on their own. Many critically ill patients requiring mechanical ventilation for a wide variety of reasons have ventilation/perfusion abnormalities as demonstrated by increases in intrapulmonary shunting and increases in dead-space ventilation. In order to diagnose and treat such conditions, it would therefore be desirable to have the capability of supplying Xenon gas or some other traceable gas for a short period to a patient without interrupting mechanical breathing assistance and without varying any of the ventilation parameters.
A number of problems have heretofore hampered efforts to solve this problem. First of all, the traceable gas is usually a radioactive isotope. Because of the obvious hazards associated with radioactive materials, any system for delivering Xenon or other radioactive gas to a patient must be designed to isolate this gas from the normal life-support mixture. Any such system must also prevent the radioactive gas from contaminating the atmosphere and harming other individuals. Furthermore, it is desirable to minimize the amount of time that the patient must be exposed to the radioactive gas in order to achieve a proper scan.
It is not possible to accomplish a ventilation scan merely by injecting Xenon gas into a conventional ventilator for a brief period. Such ventilators typically vent expired gas into the atmosphere and as previously explained this is unacceptable where radioactive gasses are utilized. Even if a closed loop ventilation system were to be utilized, an undue amount of time would be required in order to achieve the uniform concentration of radioactive gas required to achieve a good scintimetric picture. Furthermore, at the conclusion of such a scan, minute amounts of radioactive Xenon would remain in a closed loop system. The patient would be exposed to hazardous radiation for an undue amount of time before the radioactive gas could be absorbed in a trap or otherwise safely purged from the system.
Many critically ill patients cannot be switched to a second conventional ventilator for the purpose of accomplishing Xenon ventilation because of the possible dire consequences of any interruption in the mechanically assisted breathing. Even if the patient can withstand a brief interruption in mechanical ventilation, there is presently no way to assure that the second ventilator will maintain the ventilation parameters substantially constant. Variations in one or more of the ventilation parameters in switching between two independent conventional ventilators may have an adverse affect on the patient's condition.
Finally, if the breathable mixture supplied by the ventilator apparatus during the wash-in period does not have a uniform concentration of the radioactive gas the gradual spread of the illuminated pattern in the lungs shown in the scintimetric picture will reveal less useful information.